Three fractions with different average molecular mass were isolated from petroleum sulfonates by Soxhlet extraction. The molecular mass distribution was examined by electrospry ionization-mass spectra. The dynamic interfacial tension behaviors of the petroleum sulfonates and three fractions were investigated respectively. The results indicated that fraction extracted with benzene produced a greater decrease of the dynamic interfacial tension than other fractions, so the former plays a vital role in reducing interfacial tension. But petroleum sulfonates before separation exhibit better interfacial activity than all the fractions, which indicates that there exists obvious synergism between these different fractions.